Stability in development of heterozygotes and homozygotes

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Abstract
Heterozygotes show less variation between repetitive structures of a single individual, or between genetically like individuals, than do homozygotes in a number of naturally outbreeding species of plants and animals. This must generally (though not necessarily always) indicate a greater stability in development of heterozygotes than of homozygotes in these species. The greater stability has been variously interpreted as indicative of the superior genetic balance of heterozygotes which would be expected from the action of natural selection in outbreeding populations where partial heterozygosity will be the rule, and as a characteristic and inevitable property springing from a greater physiological versatility of the heterozygotic state per se. Observations on the stability properties of heterozygotes and homozygotes in inbreeding species should aid in reaching a decision between these views--a decision which is of basic importance to our understanding of the genetical structure and response to selection of both wild and domesticated populations. Nicotiana rustica naturally inbreeds to an extent which should reduce its heterozygosity to a half, or even less, of the maximum. Two experiments carried out for other purposes with this plant have provided evidence about its properties of stability in development. In respect of leaf length and capsule number there is no indication of genetical differences in stability within the material observed. In respect of plant height, $F_{1}$'s are less stable than inbred parents, where differences exist. Genetical differences also exist in respect of flowering time, but $F_{1}$'s and inbred parents show the same average stability. Thus the most, as well as the least, stable families are to be found among the inbred lines. Heterozygosity per se thus confers no greater stability of development in this partially inbreeding plant, and other inbreeding species support this conclusion. A re-examination of evidence from Drosophila melanogaster shows that maximum stability is not necessarily associated with maximum heterozygosity. Evidence obtained by Tebb and Thoday from their studies of the X-chromosome also shows that homozygotes can exhibit a developmental stability superior to that of heterozygotes. All these results are to be expected if stability in development reflects a genic balance produced by the action of natural selection, but find no obvious explanation if greater stability is referred to innate physiological properties of the heterozygotic state as such. Physiological assumptions must thus be made with caution, and in particular must not be allowed to prejudice genetical interpretation in the field of population studies.